This manual covers the Yanmar 4LHA-HTP, 4LHA-HTZP, 4LHA-DTP, 4LHA-DTZP, 4LHA-STP, and 4LHA-STZP marine diesel engines. These 4-cylinder, direct-injection diesel engines are designed for marine applications, offering robust performance and reliability. Depending on the specific model variant, these engines feature either turbocharged or naturally aspirated induction, with intercooling options available. They are engineered for efficient operation and durability, making them suitable for a wide range of marine vessels. This comprehensive Operation Manual provides essential information for the safe and effective use, maintenance, and troubleshooting of your Yanmar marine diesel engine. It includes detailed sections on safe operation, pre-operation checks, operating procedures, routine maintenance, inspection schedules, and troubleshooting guidance. System diagrams and technical specifications are also included to assist owners, mechanics, and technicians in understanding and maintaining the engine's performance and longevity.